The present invention relates generally to lead-free borophosphate glass compositions, and more particularly, to lead-free Al2O2 and B2O3 containing sub-pyrophosphate SnOxe2x80x94ZnOxe2x80x94P2O5 (SZP) sealing frit compositions suitable for bonding glass, glass-ceramic and ceramic materials, such as bonding optical waveguide fiber and fiber Bragg gratings to substrates.
Bonding frits based on SnOxe2x80x94ZnOxe2x80x94P2O5 (SZP) sealing glasses are currently used for attaching optical waveguide fibers to negative expansion xcex2-eucryptite glass-ceramic substrates to make athermal refractive index gratings or otherwise called xe2x80x9cfiber Bragg gratingsxe2x80x9d or simply xe2x80x9cgratings.xe2x80x9d Fiber Bragg gratings are well known, and are widely used in the area of optical fiber communication systems and the like. Bonding a fiber Bragg grating to a temperature compensating substrate, such as xcex2-eucryptite forms an athermal fiber Bragg grating.
The unique feature of SZP based frits is their moderate degree of bonding to the silica fiber. These frits typically possess the advantage of forming only a relatively weak to modest bond with the optical fiber. A laser is used in attaching fibers to the xcex2-eucryptite substrate. The laser heats a small amount of SZP-frit to the point of softening and a fiber is inserted. Pull-strength testing of laser-sealed gratings bonded with the standard SZP frits typically show strength values of 2.0-2.5 lbs. over a relatively wide range of laser output. Failure in pull-strength testing for gratings bonded with these SZP frits typically occurs when the fiber pulls out of the seal, which is indicative of weak interaction between fiber and frit. As a result, the fiber is not damaged or broken when subjected to the intense strains created by the typically large mismatch in coefficient of thermal expansion between the optical fiber (CTE≈0xc3x9710xe2x88x927/xc2x0 C.), and temperature compensation substrate (CTE≈xe2x88x9270xc3x9710xe2x88x927/xc2x0 C.). Other glass frit systems that have been investigated, such as PbOxe2x80x94ZnOxe2x80x94B2O3, PbOxe2x80x94B2O3, or mixed alkali-ZnOxe2x80x94P2O5 (RZP), all tend to result in fiber breakage after attachment.
Once a fiber Bragg grating has been bonded to a temperature compensating substrate, the fiber Bragg grating must be packaged with moisture getters in a hermetically sealed enclosure. The reason for doing this is primarily to protect the sealing frit from exposure to environments of high humidity and high temperature, which can affect the fiber-frit bond and lead ultimately to failure of the fiber Bragg grating by causing a center wavelength shift over the life of the grating. Historically, phosphate glasses are prone to degradation in aqueous environment. Although the sealing glass frits currently used have markedly better durability than other phosphate frits of higher P2O5 content, they still exhibit relatively poor durability to high moisture and heat. The process of hermetically sealing fiber Bragg gratings, however, adds significantly to the cost, complexity and duration of the process of manufacturing a fiber Bragg grating. Thus, if it were possible, to eliminate the need for hermeticity, one can achieve great financial savings, as well as reduce manufacture and production time of the fiber Bragg grating.
With this in mind, researchers have striven to develop alternative compositions for relatively low phosphate sealing glasses in hopes of combining the features of good durability and attachment at a relatively low softening temperature. For example U.S. Pat. No. 5,246,890 (Aitken et al.) teaches compositions for non-lead sealing glasses containing 25-50 mole percent P2O5, and SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is in the range of 1:1 to 5:1. Optionally, the glasses may contain up to 5 mole percent of SiO2; up to 5 mole percent Al2O3; and up to 20 mole percent B2O3. These SZP glasses are particularly useful as sealing glass flits for joining component parts in cathode ray tubes. The sealing frit disclosed in Aitken et al. preferably contains 29-33 mole percent of P2O5. Furthermore, Aitken et al. teaches that incorporating smaller amounts of P2O5, that is less than 29 mole percent, results in a sealing glass that exhibits an erratic and non-reproducible flow behavior in sealing operations.
Another example, U.S. Pat. No. 5,516,733 (Morena) modifies the powdered SZP sealing glass frit disclosed in U.S. Pat. No. 5,246,890 (Aitken et al.) by adding an appreciable quantity of mill additives comprising alumina and optionally zircon. A phosphate crystal phase is thermally developed in a seal produced with this modified SZP sealing glass, resulting in a substantial change in the viscosity-temperature characteristics of the seal, such that the seal remains rigid when reheated as a bake-out step. The overall effect of the mill additives on the viscosity of a fusion seal finds particular application in uniting the faceplate and funnel members to form a cathode ray tube envelope.
In a third example, U.S. Pat. No. 5,281,560 (Francis et al.) teaches non-lead sealing glasses containing 25-50 mole % P2O5, 30-70% SnO, 0-15% ZnO, wherein the mole ratio of Sn:ZnO is greater than 5:1, and an effective amount up to about 25% total of at least one oxide in the indicated proportions selected from the group consisting of 0-25% Li2O, 0-5% Na2O, 0-5% K2O, up to 20% B2O3, up to 5% Al2O3, up to 5% SiO2, and up to 5% WO3. These glasses are particularly useful as sealing glass frits in sealing material to join component parts in electrical and electronic devices.
Unlike the present invention, however, the sealing frits disclosed in Francis et al.xe2x80x94like the sealing frit disclosed in Aitken et al.xe2x80x94preferably contain 29-33 mole % P2O5. Moreover, again like in Aitken et al., Francis et al. specifically teach that lower amounts of P2O5 (less than 29 mole percent) will result in a sealing glass exhibiting an erratic and non-reproducible flow behavior in sealing operations. In their more preferred embodiment, Francis et al. disclose that P2O5, should be at or near the pyrophosphate stoichiometry, which is about 33%, as reflected in their examples.
Furthermore, persons versed in the art can recognize that a sealing frit, having the desired properties, can not be made from the compositions disclosed by Francis et al. For a sealing frit to function as we have envisioned, any significant amounts of alkali will induce crystallization by acting as an oxidizing agent to the SnO present.
Hence, the search for a sealing frit that has improved durability from exposure to high humidity and high temperature environments continues. Therefore, a great need exists for frit compositions that exhibit long term stability, durability, and resistance to attack against moisture and high humidity conditions, and to which fiber gratings can be attached without the use of a hermetic chamber.
As a consequence of the need for more durable glass compositions, a general aspect of this invention is directed to providing lead-free, Al2O2 and B2O3-containing sub-pyrophosphate SnOxe2x80x94ZnOxe2x80x94P2O5 (SZP) glass compositions suitable for use as sealing frits for bonding glass, glass-ceramic or ceramic substrates. More particularly, another aspect of the present invention pertains to a glass composition suitable for bonding optical waveguide fiber to substrates. The xe2x80x9csub-pyrophosphatexe2x80x9d category encompasses glass compositions that contain P2O5 in amounts approximately less than 32% or 33%, preferably even less than 29.5%, in terms of mole percent. The glasses of the present invention are lead-free compositions, as calculated in mole percent on an oxide basis, containing 24.5 to 29.0% P2O5, 1.0 to 5.0% B2O3, 1.0 to 2.0% Al2O3, with SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is in the range from about 5.0:1 to 12:1, (approximately 51.5 to 66.5% SnO, 5.0 to 12.0% ZnO), and 0.0 to 2.0% SiO2. Additionally, the sub-pyrophosphate glass compositions exhibit, under NMR spectroscopic analysis of 11B nuclei, a signal profile containing at least two peaks at a chemical shift in the range of approximately xe2x88x9218 to xe2x88x9225 ppm relative to aqueous boric acid. The SZP base glasses resulting from these compositions show markedly increased aqueous durability. Gratings attached with sealing frits made from such base glass compositions have shown excellent long term stability in damp or humid, hot environments, and may not require hermetic packaging during their service life.
Additional features and advantages of the invention will be set forth in the detailed description that follows. It is to be understood that both the foregoing general discussion and the following detailed description and examples provided herein are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.